This invention provides a source of EUV or SXR (EUV/SXR) Bremsstrahlung and characteristic line radiation with improved power and performance over the existing art. Such sources are particularly useful in photolithography for very fine feature size devices.
Considerable efforts are being made for the development of next generation photolithography tools. The semiconductor industry now plans to move from device feature sizes of tens of nanometers (nm) to a node, meaning a feature size for which the needed tools and materials (light sources, photomasks, photoresists, etc.) are being developed, in the EUV range of around 13.5 nm. Beyond that, plans call for nodes in the single or few nm, or SXR, range. The photon flux (or light) sources for processing wafers in these ranges have proved difficult to develop. Flux in the 100 W range and higher is needed for economical processing of wafers in the EUV node. These sources have been repeatedly delayed, however, and reliable, cost-effective sources are not yet available for producing EUV at these power levels.
The most common source for the EUV node uses high energy lasers to strike droplets of molten tin in vacuum, a process which produces intense EUV flux that is then collected by mirrors and transmitted through optical elements to the wafer being processed. EUV processing must be done in vacuum, since all nearly all materials absorb EUV. The tin droplet sources produce debris that contaminates the chambers, collection mirrors and demagnification optics used for projection lithography. They also make for extremely expensive photolithography tools, which are estimated to cost over $100 million. The power efficiency of these tools is also low; the flux reaching the wafer has only 10−4 of the input power needed for these tools. Power sources for the tin droplet EUV flux sources are therefore in the 1 MW range. Short source lifetimes and high maintenance costs are other problems with the tin droplet EUV sources. Moreover, other types of sources will be needed for progression to SXR nodes, so the massive investments in EUV sources will not be of benefit when that change occurs.
Photomasks are reflective in the EUV node, but could be transmissive in the SXR node, since light at these wavelengths can transmit through some materials. EUV and SXR can be reflected by mirrors made with multiple layers of materials, such as Mo and Si for EUV. Reflective EUV photomasks and the projection optics used in lithography tools make use of such mirrors.
It is known in the art that EUV and SXR can be produced in a manner similar to the way harder x-rays are produced in a standard x-ray tube, i.e. by hitting an anode target with an accelerating electron beam from a cathode. Several materials, such as Si, SiC, Be, Mo, Cu and alloys thereof, can produce characteristic line, and Bremsstrahlung, radiation in the 10-15 nm range, and other materials can be used as anodes for SXR. In particular, Si and SiC produce characteristic line radiation around 13.5 nm. Shell electrons of atoms in the target are ejected from their shell through electron impact excitation by the electrons from the cathode. The unoccupied shell is quickly refilled, which results in the emission of a photon from the atom with an energy characteristic of that shell. The radiation is isotropic and independent of the direction of the colliding electron's momentum direction.
Point sources of EUV have been developed in which a Si or SiC anode target is used in a standard x-ray tube configuration, i.e. with an e-beam from a cathode accelerating into an angled anode in an evacuated tube. These, however, have had low (10−5) power efficiency, with the best efficiencies produced by an accelerating voltage between cathode and anode of 8 kV to 10 kV. A major reason for this low power efficiency is that the target material absorbs the EUV photons. This is because the characteristic line and other photons are generated at some distance below the surface of the target material. As the accelerating electron voltage is increased the electrons penetrate deeper into the target and the probability impacting one of the electrons in the target material increases. This corresponds to an increase in EUV flux radiating off the target. The increase in flux, however, only increases to a certain point and then starts to decrease. This is because the electrons are then penetrating so deep into the material that the bulk of the EUV photons are absorbed before they can reach the surface of the target. Moreover, a single electron beam cannot impart enough power to the anode for this source to be useful in photolithography, or even in measurement tools. The fundamental process, however, has a number of advantages over the tin droplet sources now being developed, such as the lack of debris and compatibility with use in a vacuum chamber, which also obviates the need for a glass tube.
A need therefore exists for more efficient, more reliable and less expensive EUV and SXR sources that can produce higher flux power, especially for use in photolithography tools.